Los domos sálicos de Fuerteventura

Abstract

In the island of Fuerteventura there is a group of trachytic rock outcrops crossing the Basal Complex and associated with the Old basaltic Miocene Series in which they are in many cases clearly intrusive. These outcrops, scarttered all over island, stand out in the landscape forming domeane like edificies. This explains, the name "Roques" generally used to designate them. Starting with a morphological study and estimations of its emplacement mechanisms, a classification of volcanic domes applicable to these extrusions in the Canarian Archipiélago is attempted. This classification reflects the existence of overflown domes or spilled domes, plugs and cumulus domes of endogenous growth. There are some very eroded edificies which are difficult to classify. In some cases, fragmentary elements are found, indicating that explosive phenomena took place during their emplacement. A petrographic study of their materials has also been performed, along with 17 new analysis of major and minor elements enabling us to accomplish a comparative study of their geochemical characteristics. In all these cases, we are dealing with materials of saturated alcaline series, mainly trachytic, with normative quartz, although there are some samples that are classified as riolites in the TAS diagram. Within each dome the degree of geochemical variability is sometimes very strong and it is related to different materials from different extrusive periods. Strongest variations correspond to domes in the Jandia peninsula, through they are not very different from those of the rest of the island. When comparing geochemical characteristics of these domes with those of sienitic materials of the so called "circular formations" on the Betancurian Massif, the main differences are higher peralcalinity of the domes and presence of normative nepheline in the sienitic materials. However, a progressive transition may be found between both of them.